A circuit board has an electronic circuit as its component, and a circuit board including elements such as thin-film transistors (TFTs), for example, is widely used as a component of an electronic device such as a liquid crystal display device, an organic electroluminescent display device, and a solar battery.
Hereinafter, a circuit board is illustrated based on an example of a circuit configuration of a TFT array substrate constituting a TFT-driven liquid crystal display panel. A TFT array substrate has a pixel circuit that has a structure in which TFTs (switching elements) are disposed at intersections of m×n matrix wirings with m rows of scanning lines and n columns of signal lines. The drain wirings of the TFTs are connected to picture element electrodes. Also, peripheral circuits such as a scanning driver IC and a data driver IC are respectively connected to the gate wirings and source wirings of the TFTs. For the wirings, low-resistance aluminum wirings are often used.
A circuit is affected by the performance of TFTs formed on a TFT substrate. That is, the performance of TFTs formed on a TFT substrate varies according to the material of the TFTs, and thus the circuit formed on the TFT substrate affects whether the circuit can be driven by the TFTs formed on the circuit board, whether the circuit size is not large, whether the yield does not decrease, and the like. Although a—Si (amorphous silicon) is used in many cases for conventional circuit boards in that TFTs can be easily formed at low cost, a—Si has low electron mobility. For this reason, a material better in electronic properties has been desired.
Regarding other semiconductor compounds usable for a channel layer of a TFT, Patent Document 1, for example, discloses a thin-film transistor that has an oxide semiconductor containing one element selected from In, Ga, and Zn as its channel layer.